1. Field of the Invention
This invention relates generally to free piston machines, and more particularly to an improved piston for a free piston Stirling cycle machine wherein the improvement is the structure of the core for a multi-piece piston and a method for fabricating the improved piston.
2. Description of the Related Art
Free piston Stirling cycle machines have been used for many years in applications such as engines, coolers, and cryocoolers. In a Stirling machine, a working gas is confined in a work space comprised of an expansion space and a compression space. The working gas is alternately expanded and compressed by the reciprocation of a free piston or by variations in the working gas temperature. The working gas is shuttled between the compression space and the expansion space which are connected in fluid communication through a heat accepter, regenerator and heat rejecter. In a typical configuration, the shuttling is accomplished by a displacer connected to a displacer rod, the displacer being driven by variations in the working gas pressure.
Free piston Stirling cycle machines are designed to have relatively small gaps between the piston and the interfacing wall of the cylinder in which the piston reciprocates in order to minimize leakage between them. Additionally, some free piston Stirling machines have a reciprocating displacer rod that extends sealingly and slidably through a central bore in the piston into connection to a planar spring. The central bore and the displacer rod are also machined to have relatively small gaps for the same reason. To minimize wear of the interfacing, sliding surfaces around and within the piston, the gaps between the interfacing surfaces are continuously lubricated. The preferred method for achieving lubrication is through the use of gas bearings. A free piston machine employing a gas bearing lubrication system similar to the gas bearing system used with embodiments of the present invention is described in U.S. Pat. No. 6,293,184 to Unger, which is herein incorporated by reference. In these gas bearing lubrication systems, working gas is supplied from the work space through a check valve to a plenum that functions as a reservoir of pressurized working gas within the piston for use in the gas lubrication. The gas is distributed from the plenum through a network of annular, longitudinal and radial gas bearing passageways, including gas flow rate metering restrictions, to the piston surfaces that slide against other surfaces.
In order to facilitate forming the network of gas bearing passageways within the piston, the piston is typically fabricated as a two-piece body comprising a sleeve and a core. The core is coaxially within and sealed to the sleeve as shown and described in the above cited U.S. Pat. No. 6,293,184 to Unger. This multi-piece piston configuration is useful because it allows many of the gas bearing passageways and the plenum to be formed on the exterior surface of the core. That is advantageous because the exterior surface is more accessible and more easily machined through processes such as drilling, grinding, etching, and turning on a lathe before the pieces are assembled to form the piston. The core is then pushed into the sleeve, and the outer surface of the core is sealed to the inner surface of the sleeve by shrink-fitting or other sealing processes.
In Stirling machines having a displacer rod axially reciprocating through the piston, a central bore must be formed through the core of the piston to receive the displacer rod. This central bore should be coaxial with the cylinder wall in which the piston reciprocates, so that piston reciprocation and displacer reciprocation occur along parallel, coaxial paths. However, the core of a typical multi-piece piston is too long for the central bore to be accurately machined in a single boring operation from one end of the core and maintain a sufficient concentricity of the bore and the outside diameter of the core. The reason is that the axis of a bore becomes inaccurate after the length/diameter ratio of the bore exceeds 3 or 4 as a result of the long, overhung boring bar during the boring operation.
It is possible to machine two separate, shorter bores into the core, one from each end of the core, meeting at a point near the core's longitudinal center and creating a single, long bore extending along the entire length of the core. This can be done by clamping the workpiece in a chuck, turning the entire outside diameter and boring half way through the core. Then the core is removed from the chuck, turned around and its opposite end clamped in the chuck and then boring from the opposite end to meet the initial bore. However, coaxial alignment of two bores entering opposite ends of the core is very difficult to achieve because of alignment imperfections associated with each clamping of an outside diameter in a chuck. Consequently, by resorting to clamping the core twice, once at each end, the alignment variations between the two result in variations in the alignment of the two bores each one formed when a different end was clamped. Misalignment of the axes of the two bores results in one or more intervals along the length of the piston in which the cylindrical outer wall of the piston and the cylindrical interior wall of the central bore are not concentric. Such compromises of the coaxiality and the concentricity of the central bore relative to the peripheral cylindrical outer surface of the outer sleeve degrade the performance of the piston and the displacer and impose a lower limit on the permissible size of the gap between the relatively sliding surfaces.
It is therefore an object and feature of the invention to provide a multi-piece piston having improved coaxiality and concentricity of the central bore and the outer cylindrical surface of the outer sleeve along the entire length of the piston, and to provide a method for making the same, for a free piston machine.